Armageddon is a bad model for saving Earth, but nukes might still be useful.

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In 2013, a small asteroid exploded in the atmosphere over Chelyabinsk, Russia. The sonic boom from the event sent more than a thousand people to the hospital, mostly from flying glass from shattered windows. The Chelyabinsk meteor was a relatively small chunk of space rock—asteroid researchers think it was probably about 20 meters (66 feet) across—but exploding over a city made it a noteworthy event. It's probable many similar asteroids hit Earth on a regular basis, but most don't happen to fly over metropolitan areas; they fall into the ocean or over lightly populated regions.

However, Earth has played target in the cosmic darts tournament before. Meteor Crater in Arizona, the Tunguska impact in Siberia in 1908, and most famously the Chicxulub asteroid in Mexico (which played a part in the extinction of the dinosaurs) are just three of many known examples. That's why many people are looking at viable options for planetary defense: destroying or turning asteroids aside before they can hit Earth. And planetary defense is one reason the United States' National Nuclear Safety Administration (NNSA) has given for not destroying some of its surplus nuclear warheads.

It's easy to be cynical about American nuclear weapons policy, especially now that we're decades since the end of the Cold War. Debates over nuclear winter, mutually assured destruction, and the like feel very distant. So reports that the US wasn't following the stated schedule for decommissioning nukes in the name of planetary defense triggered the skeptical radar, not least since The Atlantic, The Wall Street Journal, and other sources made it sound like the plan was to blow asteroids to smithereens.

There are many good reasons to doubt the wisdom of such a strategy, but it turns out this initial impression—and the impression given by many published articles—was wrong. The real plan is a lot less problematic than trying to obliterate an asteroid.

As a result, there's reason to be less cynical about the prospect of nuking asteroids, though there are still some open questions and fierce debate over planetary defense. To see the fuller picture, it's necessary to look at the risks of asteroid impacts, what we know about asteroids themselves, and what that means for the prospect of pushing them around. So let's examine whether the stated goals of stockpiling nukes are consistent with asteroid mitigation; sadly Bruce Willis will not be involved.

Death from above

Most asteroids and the smaller chunks of rock we call meteoroids are Chelyabinsk-scale threats, but a significant number are bigger—and not all of them are far away. While many big asteroids reside in the Main Belt (sometimes simply called the "asteroid belt") between Mars and Jupiter, researchers have identified a large number of near-Earth asteroids (NEAs), of which about 1,563 are deemed "potentially hazardous." NEAs are asteroids that orbit in a range a little closer to us than Mars. However, while a number of these NEAs cross Earth's orbit frequently—the "Apollo" and "Aten" groups of asteroids—very few large specimens come anywhere close to us. And none of them present an immediate danger to us. After all, the Solar System is a big place and Earth is relatively small.

For that reason, experts are divided on how much we should worry about asteroid impacts right now. The risk is low for the next few decades, but the potential damage is sufficiently high even for small impacts that some think we should focus a lot of effort on mitigation. The dinosaur-killing Chicxulub asteroid may have been 10 kilometers (six miles) across, but we don't need one that big to wreak serious havoc.

So if we want to take the long view, there is a persistent danger. Gravity from Jupiter and (to a lesser degree) Saturn can kick asteroids out of the Main Belt. Some of those are ejected from the Solar System entirely or fall into the Sun, but others end up in shallower orbits, where they might become new NEAs. The rate at which the gas giants are creating NEAs may be slow, but we're well advised to keep watch anyway. If we spotted an asteroid today that could be on a collision course with Earth, say within the next 30 years, it gives us time to prepare now rather than closer to the time of potential disaster.

"The dinosaurs didn't have a space program, much less telescopes, so it didn't end very well for the dinosaurs," says Alessondra Springmann, a planetary scientist who studies asteroids at the University of Arizona. "Hopefully we can find asteroids before they find us."

Asteroid researchers would also like to see them spotted because they want to study the structure of NEAs. "Planets are formed out of the same things the asteroids are formed out of," says Springmann. "But planets have been heated up, they've melted, the planets have surface processes, and Venus, Earth, Mars, the outer planets all have atmospheres." Asteroids, by contrast, are relatively pristine: "They're a whole source of information about Solar System as it formed." NEAs are particularly nice for the practical reason that they're close to us. We don't need to fly space probes (or presumably humans) out past the orbit of Mars to study them.

One fascinating discovery has been made by early NEA studies. "About 15 percent of near-Earth asteroids larger than 200 meters [656 feet] have moons," says Springmann. By measuring the orbits of those moons, researchers can find the mass of the asteroids they orbit using Kepler's Third Law (the same way astronomers measured the mass of the Sun or Jupiter). Combining the mass with radar measurements of the size of NEAs, astronomers have obtained the overall density of the asteroids.

The result: many asteroids are "rubble piles" (more formally known as "gravitational aggregates"), collections of smaller rocks and grains held together by gravity and molecular forces derived from static electricity. With some densities just greater than water, "they're more like conglomerations of styrofoam rather than a big type of boulder," says Springmann.

The aggregate nature has some interesting consequences. According to our theoretical understanding of these bodies, sunlight can heat one side of these asteroids slightly more than the other, increasing their spin until they literally fragment. At least one seems to have broken up entirely, but in less drastic cases, a smaller chunk falls off to make one of the moons astronomers have observed. And we might be able to exploit the effect sunlight uses to steer asteroids away from Earth.

Asteroid variety

Cristina Thomas of Goddard Space Flight Center notes that the sheer variation in asteroid properties makes classification hard. Asteroids range from very dark—blacker than a chalkboard—to light-colored, reflecting as much as 50 percent of sunlight back. A major way to catalog asteroid diversity is by looking at the mineralogy of meteorites, which are smaller chunks that have fallen to Earth, but which are fragments of the same types of rock constituting their larger asteroid cousins.

That's why all of this information is incredibly relevant for planetary defense. Rubble pile asteroids would require different mitigation techniques than solid, rocky, monolithic asteroids, but just because they're loose aggregates doesn't mean they can be easily broken up. "If you smacked an asteroid and it fell apart, it's likely that given enough time, it would re-accrete unless you had a very, very large offense," says Cristina Thomas of NASA's Goddard Space Flight Center. The tool we need to reduce the threat is more a metaphorical shovel than a hammer.

Of course, even if we successfully broke an asteroid into pieces, we'd need to make sure those pieces don't all hit us and cause as much destruction as the original asteroid. From the science fiction films Armageddon and Deep Impact, you might get the idea that we shouldn't close our eyes or fall asleep the best strategy is to use nuclear weapons to blow the threatening space rock to smithereens. This idea isn't off the table completely, but it must be kept as a last resort, something to be considered only if there isn't enough time to put together something less risky to Earth. And, somewhat surprisingly, there's another possible use of nuclear weapons that is less risky.

It's easy to be cynical about American nuclear weapons policy, especially now that we're decades since the end of the Cold War.

Perhaps from a Western perspective the Cold War has long since ended, but recent and future events may require a revision of that assessment. Being oblivious to a threat that never really went away is not the same as the threat being gone, though it's easy to forget when it is no longer highly visible and overt. Though that too, is changing.

Since the asteroid would be headed our way, why not steer it into orbit and mine it?

I'm just guessing, but possible difficulties include:1. having enough control over the trajectory to get it into earth orbit2. getting it into orbit without breaking it up3. there being economically-worthwhile quantities of material in the asteroid (particularly if it's a 'rubble pile' type).

It seems likely that the practical difficulties (and ensuing costs) of getting an object into earth orbit would outstrip any economic benefits having it there might produce. It would be an amazing feat, and totally awesome for science, however.

Again, I could be wrong about any/all of this. I'm a liberal arts grad with an a deep and abiding interest in space, rather than anyone with directly relevant expertise.

I'm more partial to the use of focusing mirrors to heat up a spot on the asteroid and create a sustained jet, basically continuous nuclear ablation without the nuke. Another option I really like is to simply park a spacecraft next to the asteroid and have its gravity act as a tug to pull the thing out of the "keyhole."

Looks like we won't get rid of nukes soon. That option has passed with Vladimir openly wondering if he should throw one in case NATO soldiers protect a Nato country like Estonia. So I think that isn't the big problem.

I suspect that it would take more than a single charge or engine to be effective. Wouldn't a single charge or single propulsion system more likely to start it spinning than shift it's course? Unless calculated accurately and that would be math and science way beyond my abilities.

The amount of energy imparted by an explosion is at best an inverse square relation (if not inverse cube). Why detonate hundreds of meters away, instead of single digits? Surely our control systems are fast and accurate enough for that these days.

Hmm... okay, thought of a counter-reason against it. If it's a rubble pile asteroid, it might be safer to hit it with a more uniform force, so that the entire thing is moved away in a single unit. A direct hit would shatter it and pretty much guarantee that some fraction of the rubble would hit earth.

Whereas for a solid asteroid, a lander with thrusters is more effective than a nuke from any distance.

The amount of energy imparted by an explosion is at best an inverse square relation (if not inverse cube). Why detonate hundreds of meters away, instead of single digits? Surely our control systems are fast and accurate enough for that these days.

I think that, if it were closer, it would shatter the asteroid rather than creating the desired plasma jet.

Fantastic article - I'm happy Ars is doing weekend long-format articles, when it's nice to take a break from the "news" and enjoy some in-depth writing & analysis.

Pet peeve, wearing my ecologist hat - dinosaur extinction is a well-worn but deceptive trope. The truth is that:1. Almost all species that ever existed are now extinct. In fact, the average "lifespan" of a species is quite short, on the order of a few million years?2. The dinosaurs left descendants in the form of birds.

That a particular species of dinosaur is now extinct is unsurprising. The noteworthy feature is that many different species feom one clade went extinct at once in a "Great Extinction Event". Ironically, best estimates indicate that Earth is currently experiencing another such massive global extinction event caused by us humans.

The apples-to-apples comparison of dinosaurs w/modern earth would be "asteroid causes extinction of most/all mammals". But a huge number of mammal species have recently gone extinct or are threatened, and human land use is the asteroid.

Just to note the concept of using nukes to nudge asteroids isn't new even in media. The adventure game The Dig's first and simplest puzzle is placing small nukes on the surface of a hard rocky asteroid then detonate them to knock it off trajectory.

Then you go inside and it's a spaceship that takes you to a mostly dead world but still. Granted, no space detonation and ablation there.

Since the asteroid would be headed our way, why not steer it into orbit and mine it?

It is not because it is headed your way that it is easy to capture : it's like catching a (big) bullet.

To put it into earth orbit, you would need to slow it down by several hundreds if not thousands m/s, which is A LOT harder than the few cm/s deflection discussed in the article : earth orbit the sun at roughly 30 km/s, the asteroid has similar speed. They both orbit in the same direction so the delta-v is most probably less (maybe not if the asteroid orbit is very elliptic), but nonetheless it is huge compared to the one needed for deviation.

Since the asteroid would be headed our way, why not steer it into orbit and mine it?

It is not because it is headed your way that it is easy to capture : it's like catching a (big) bullet.

To put it into earth orbit, you would need to slow it down by several hundreds if not thousands m/s, which is A LOT harder than the few cm/s deflection discussed in the article : earth orbit the sun at roughly 30 km/s, the asteroid has similar speed. They both orbit in the same direction so the delta-v is most probably less (maybe not if the asteroid orbit is very elliptic), but nonetheless it is huge compared to the one needed for deviation.

As the object came from beyond Earth's orbit, it will be going at - at least - escape velocity simply thanks to gravity.

Without controlled gravity assists from the Moon (the Moon is VERY, VERY useful here, as it can absorb the excess velocity and won't really notice due to it being enormous) the amount of velocity you'd need to shed from the asteroid would, in the case of a 500 m stony chondrite, exceed the gravitational binding energy of the asteroid.

Your attempts to slow it down enough to enter Earth orbit would actually disrupt the asteroid.

I'm pretty sure that if an asteroid was heading towards earth that was large enough to threaten the end of humanity, no one is going to give a damn about what words on a treaty say. If the math says the asteroid is on a collision course, most everyone in the world is just going to say go for it. Who cares if the technique is untested? Not trying to defect it is going to have the same end result if you try to defect it and it fails.

Since the asteroid would be headed our way, why not steer it into orbit and mine it?

It is not because it is headed your way that it is easy to capture : it's like catching a (big) bullet.

To put it into earth orbit, you would need to slow it down by several hundreds if not thousands m/s, which is A LOT harder than the few cm/s deflection discussed in the article : earth orbit the sun at roughly 30 km/s, the asteroid has similar speed. They both orbit in the same direction so the delta-v is most probably less (maybe not if the asteroid orbit is very elliptic), but nonetheless it is huge compared to the one needed for deviation.

As the object came from beyond Earth's orbit, it will be going at - at least - escape velocity simply thanks to gravity.

Without controlled gravity assists from the Moon (the Moon is VERY, VERY useful here, as it can absorb the excess velocity and won't really notice due to it being enormous) the amount of velocity you'd need to shed from the asteroid would, in the case of a 500 m stony chondrite, exceed the gravitational binding energy of the asteroid.

Your attempts to slow it down enough to enter Earth orbit would actually disrupt the asteroid.

Well, you don't want to remove all those 11 km/s unless you want to crash the asteroid. LEO is 7.8 km/s, so you don't want to remove more than 3.2 km/s in order to keep it in orbit. To sum up you need to remove most of the delta-v when out of earth sphere of influence, plus a maximum of 3.2 km/s into earth sphere of influence depending of the orbit you want to achieve.

Since the asteroid would be headed our way, why not steer it into orbit and mine it?

It is not because it is headed your way that it is easy to capture : it's like catching a (big) bullet.

To put it into earth orbit, you would need to slow it down by several hundreds if not thousands m/s, which is A LOT harder than the few cm/s deflection discussed in the article : earth orbit the sun at roughly 30 km/s, the asteroid has similar speed. They both orbit in the same direction so the delta-v is most probably less (maybe not if the asteroid orbit is very elliptic), but nonetheless it is huge compared to the one needed for deviation.

As the object came from beyond Earth's orbit, it will be going at - at least - escape velocity simply thanks to gravity.

Without controlled gravity assists from the Moon (the Moon is VERY, VERY useful here, as it can absorb the excess velocity and won't really notice due to it being enormous) the amount of velocity you'd need to shed from the asteroid would, in the case of a 500 m stony chondrite, exceed the gravitational binding energy of the asteroid.

Your attempts to slow it down enough to enter Earth orbit would actually disrupt the asteroid.

Well, you don't want to remove all those 11 km/s unless you want to crash the asteroid. LEO is 7.8 km/s, so you don't want to remove more than 3.2 km/s in order to keep it in orbit. To sum up you need to remove most of the delta-v when out of earth sphere of influence, plus a maximum of 3.2 km/s into earth sphere of influence depending of the orbit you want to achieve.

You probably would want to remove more than 11 km/s. The object was not stationary relative to Earth, it had its own heliocentric orbital velocity, which Earth adds that 11 km/s (ish) to. Most asteroids, looking up from an impact simulator, would be doing 30-50 km/s on contact with the upper atmosphere, so that's 20-40 km/s before Earth even enters the picture.

There is nothing illegal or treaty breaking about nuclear explosions in space. We can't store nukes in space, or test them, but they can be used. Most of this was covered under the 1967 Outer Space treaty, and then reinforced with Comprehensive Test Ban Treaty(CTBT). The CTBT is just more recent and has more signatories.

Since the asteroid would be headed our way, why not steer it into orbit and mine it?

It is not because it is headed your way that it is easy to capture : it's like catching a (big) bullet.

To put it into earth orbit, you would need to slow it down by several hundreds if not thousands m/s, which is A LOT harder than the few cm/s deflection discussed in the article : earth orbit the sun at roughly 30 km/s, the asteroid has similar speed. They both orbit in the same direction so the delta-v is most probably less (maybe not if the asteroid orbit is very elliptic), but nonetheless it is huge compared to the one needed for deviation.

As the object came from beyond Earth's orbit, it will be going at - at least - escape velocity simply thanks to gravity.

Without controlled gravity assists from the Moon (the Moon is VERY, VERY useful here, as it can absorb the excess velocity and won't really notice due to it being enormous) the amount of velocity you'd need to shed from the asteroid would, in the case of a 500 m stony chondrite, exceed the gravitational binding energy of the asteroid.

Your attempts to slow it down enough to enter Earth orbit would actually disrupt the asteroid.

Well, you don't want to remove all those 11 km/s unless you want to crash the asteroid. LEO is 7.8 km/s, so you don't want to remove more than 3.2 km/s in order to keep it in orbit. To sum up you need to remove most of the delta-v when out of earth sphere of influence, plus a maximum of 3.2 km/s into earth sphere of influence depending of the orbit you want to achieve.

You probably would want to remove more than 11 km/s. The object was not stationary relative to Earth, it had its own heliocentric orbital velocity, which Earth adds that 11 km/s (ish) to. Most asteroids, looking up from an impact simulator, would be doing 30-50 km/s on contact with the upper atmosphere, so that's 20-40 km/s before Earth even enters the picture.

Yes, it is exactly what I was saying in my first post, where I completely ignored earth gravity, since it is not really important compared to the relative speeds into sun orbit (and also it is possible to orbit earth at the limit of its sphere of influence)

"The dinosaurs didn't have a space program, much less telescopes, so it didn't end very well for the dinosaurs," says Alessondra Springmann, a planetary scientist who studies asteroids at the University of Arizona. "Hopefully we can find asteroids before they find us."

And when there is an asteroid strike, there will always be groups blaming it on the Democratic party voters for not supporting cutting taxes and science programs. And yes, I do include Third-Way Dem pols in these groups.

The concept for nuclear ablation was actually used for plans in an experimental rocket. Nuclear propulsion would have made rockets so efficient that we probably could have sent a man to mars by now. Unfortunately the nuclear space ban prevents us from using these kinds of rockets in space.

The amount of energy imparted by an explosion is at best an inverse square relation (if not inverse cube). Why detonate hundreds of meters away, instead of single digits? Surely our control systems are fast and accurate enough for that these days.

"It sometimes becomes necessary to use a nuclear device simply because the current launch vehicles aren't capable of transporting enough mass to bring a kinetic impactor that would be able to accomplish the deflection."

Quote:

"The dinosaurs didn't have a space program, much less telescopes, so it didn't end very well for the dinosaurs," says Alessondra Springmann, a planetary scientist who studies asteroids at the University of Arizona. "Hopefully we can find asteroids before they find us."

The concept for nuclear ablation was actually used for plans in an experimental rocket. Nuclear propulsion would have made rockets so efficient that we probably could have sent a man to mars by now. Unfortunately the nuclear space ban prevents us from using these kinds of rockets in space.

There's also the small matter of how you get so many thousands of bombs up off the ground in the first place, not to mention what you make the blast plate out of that it doesn't fall apart after you're underway and being bombarded by hard radiation.

Sending an actual 'mass' in the form of a missile/spaceship/slug/whatever to an asteroid sounds a bit primitive 'already' to me.

I would think it will be much cheaper to use a laser or combined mirrors using focussed rays of the sun to heat up one side of the asteroid and make it drift off a bit (and hopefully it's not rotating ((fast)), otherwise you'd have to pulse the laser to hit the same spot on the asteroid every time).

Although focussing significant and effective energy on a spot over some million miles away might be another challenge by itself.

Sending an actual 'mass' in the form of a missile/spaceship/slug/whatever to an asteroid sounds a bit primitive 'already' to me.

I would think it will be much cheaper to use a laser or combined mirrors using focussed rays of the sun to heat up one side of the asteroid and make it drift off a bit (and hopefully it's not rotating ((fast)), otherwise you'd have to pulse the laser to hit the same spot on the asteroid every time).

Although focussing significant and effective energy on a spot over some million miles away might be another challenge by itself.

How much energy would it take to heat it up, and how easy is it to target an object moving at such a velocity compared to us, and what are the implications of that kind of energy shooting through our atmosphere - or are you suggesting a network of satellites?

This sounds a lot like the idea proposed in the original Project Icarus, a class project at MIT back in 1967. Their solution was written up in a book published by MIT Press and included inputs from aeronautics and astronautics students, civil and mechanical engineers, physicists, and students in nuclear engineering. Their idea was to use a nuclear explosion to crater the asteroid, ejecting mass at high velocity to change the asteroid's course. They used nuclear yield data, and extrapolated the cratering and ejection from earth based experiments with high explosives and nuclear weapons. It was Newton's Third Law, action and reaction. They even accounted for gravitational recapture and discussed the problem of aggregate asteroids, as back then no one knew if asteroids were solid or loosely bound rock piles.

The plan involved a series of such nuclear explosions, since one would not be sufficient. This meant some serious logistical work since launch capacity was limited. I think they proposed using Saturn 1 rocket motors. These would have been launched into orbit as upper stages and then optimally launched from orbit to intercept the asteroid. These motors used liquid oxygen which introduced another wrinkle. A modern solid fuel rocket could be kept ready to launch in orbit for long periods, but liquid oxygen would evaporate during staging. If anything, this is an easier problem now than it was back then. All that Cold War, the rockets must be ready to launch at all times at a moment's notice may pay off. Who says there is no such thing as progress.